Input: atkbd - expose function row physical map to userspace

Certain keyboards have their top-row keys intended for actions such as
"Browser back", "Browser Refresh", "Fullscreen" etc as their primary mode,
thus they will send scan codes for those actions. Further, they don't
have a dedicated "Fn" key so don't have the capability to generate
function key codes (e.g. F1, F2 etc..).  However in this case, if
userspace still wants to "synthesize" those function keys using the top
row action keys, it needs to know the physical position of the top row
keys. (Essentially a mapping between usage codes and a physical location
in the top row).

This patch enhances the atkbd driver to receive such a mapping from the
firmware / device tree, and expose it to userspace in the form of a
function-row-physmap attribute. The attribute would be a space separated
ordered list of physical codes, for the keys in the function row, in
left-to-right order.

The attribute will only be present if the kernel knows about such mapping,
otherwise the attribute shall not be visible.

Signed-off-by: Rajat Jain <rajatja@google.com>
Link: https://lore.kernel.org/r/20200427210259.91330-2-rajatja@google.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/atkbd.c

@@ -24,6 +24,7 @@
 #include <linux/libps2.h>
 #include <linux/mutex.h>
 #include <linux/dmi.h>
+#include <linux/property.h>
 
 #define DRIVER_DESC	"AT and PS/2 keyboard driver"
 
@@ -63,6 +64,8 @@ static bool atkbd_terminal;
 module_param_named(terminal, atkbd_terminal, bool, 0);
 MODULE_PARM_DESC(terminal, "Enable break codes on an IBM Terminal keyboard connected via AT/PS2");
 
+#define MAX_FUNCTION_ROW_KEYS	24
+
 /*
  * Scancode to keycode tables. These are just the default setting, and
  * are loadable via a userland utility.
@@ -230,6 +233,9 @@ struct atkbd {
 
 	/* Serializes reconnect(), attr->set() and event work */
 	struct mutex mutex;
+
+	u32 function_row_physmap[MAX_FUNCTION_ROW_KEYS];
+	int num_function_row_keys;
 };
 
 /*
@@ -283,6 +289,7 @@ static struct device_attribute atkbd_attr_##_name =				\
 	__ATTR(_name, S_IRUGO, atkbd_do_show_##_name, NULL);
 
 ATKBD_DEFINE_RO_ATTR(err_count);
+ATKBD_DEFINE_RO_ATTR(function_row_physmap);
 
 static struct attribute *atkbd_attributes[] = {
 	&atkbd_attr_extra.attr,
@@ -292,11 +299,42 @@ static struct attribute *atkbd_attributes[] = {
 	&atkbd_attr_softrepeat.attr,
 	&atkbd_attr_softraw.attr,
 	&atkbd_attr_err_count.attr,
+	&atkbd_attr_function_row_physmap.attr,
 	NULL
 };
 
+static ssize_t atkbd_show_function_row_physmap(struct atkbd *atkbd, char *buf)
+{
+	ssize_t size = 0;
+	int i;
+
+	if (!atkbd->num_function_row_keys)
+		return 0;
+
+	for (i = 0; i < atkbd->num_function_row_keys; i++)
+		size += scnprintf(buf + size, PAGE_SIZE - size, "%02X ",
+				  atkbd->function_row_physmap[i]);
+	size += scnprintf(buf + size, PAGE_SIZE - size, "\n");
+	return size;
+}
+
+static umode_t atkbd_attr_is_visible(struct kobject *kobj,
+				struct attribute *attr, int i)
+{
+	struct device *dev = container_of(kobj, struct device, kobj);
+	struct serio *serio = to_serio_port(dev);
+	struct atkbd *atkbd = serio_get_drvdata(serio);
+
+	if (attr == &atkbd_attr_function_row_physmap.attr &&
+	    !atkbd->num_function_row_keys)
+		return 0;
+
+	return attr->mode;
+}
+
 static struct attribute_group atkbd_attribute_group = {
 	.attrs	= atkbd_attributes,
+	.is_visible = atkbd_attr_is_visible,
 };
 
 static const unsigned int xl_table[] = {
@@ -1121,6 +1159,22 @@ static void atkbd_set_device_attrs(struct atkbd *atkbd)
 	}
 }
 
+static void atkbd_parse_fwnode_data(struct serio *serio)
+{
+	struct atkbd *atkbd = serio_get_drvdata(serio);
+	struct device *dev = &serio->dev;
+	int n;
+
+	/* Parse "function-row-physmap" property */
+	n = device_property_count_u32(dev, "function-row-physmap");
+	if (n > 0 && n <= MAX_FUNCTION_ROW_KEYS &&
+	    !device_property_read_u32_array(dev, "function-row-physmap",
+					    atkbd->function_row_physmap, n)) {
+		atkbd->num_function_row_keys = n;
+		dev_dbg(dev, "FW reported %d function-row key locations\n", n);
+	}
+}
+
 /*
  * atkbd_connect() is called when the serio module finds an interface
  * that isn't handled yet by an appropriate device driver. We check if
@@ -1184,6 +1238,8 @@ static int atkbd_connect(struct serio *serio, struct serio_driver *drv)
 		atkbd->id = 0xab00;
 	}
 
+	atkbd_parse_fwnode_data(serio);
+
 	atkbd_set_keycode_table(atkbd);
 	atkbd_set_device_attrs(atkbd);